1. Field of the Invention
The present invention relates to a zoom lens system having a wide angle-of-view of approximately 80° at the short focal length extremity and having a zoom ratio of approximately 4, to be used in a light-weight, compact digital camera. The present invention also relates to an electronic imaging apparatus which uses such a zoom lens system.
2. Description of Related Art
In recent years, further miniaturization of digital compact cameras due to further miniaturization of electronic components has been progressing, and further miniaturization of the photographing optical system is also in demand. On the other hand, in order to further enhance the added value of a camera, there is also a high demand to increase both the angle-of-view and the zoom ratio, which tend to increase the size of the optical system. Accordingly, it is a challenging problem to achieve the best possible balance between miniaturization and high specifications.
A negative-lead type lens system is often used in a zoom lens system having a zoom ratio of approximately 3 as a zoom lens system for a compact digital camera. Since a negative-lead type lens system can widen the angle-of-view at the short focal length extremity and miniaturize the lens system, especially the diameter of the most object-side lens group, the negative-lead type lens system is suitable for the so-called retractable zoom lens camera in which the lens groups are accommodated by reducing the distances therebetween when the lens groups are being retracted to an accommodating position.
Furthermore, due to the need to position the exit pupil sufficiently far away from the imaging plane, a three-lens-group zoom lens system, i.e., a first lens group having a negative refractive power (hereinafter, a negative first lens group), a second lens group having a positive refractive power (hereinafter, a positive second lens group)and a third lens group having a positive refractive power (hereinafter, a positive third lens group) is often employed (e.g., Japanese Unexamined Patent Publication Nos. 2005-70696 and 2005-37727).
In the case of a zoom lens system having a zoom ratio of 4 or more, a positive-lead lens system is often employed.
Such a positive-lead type zoom lens system is suitable for reducing the overall length of the zoom lens system; however, the diameter of the first lens group (the most object-side lens group) is very large, which is not suitable for the so-called retractable zoom lens camera having a multi-stage retractable lens barrel.
In the aforementioned Japanese Unexamined Patent Publication Nos. 2005-70696 and 2005-37727, relatively small optical systems having a zoom ratio of approximately 3 are disclosed; however, the optical systems disclosed therein do not attain a sufficient zoom ratio, and do not achieve a sufficient reduction in costs.
Generally, a negative-lead type lens system is desirable for a camera with a retractable zoom lens system. However, if the zoom ratio is increased up to approximately 4, the overall length of the zoom lens system tends to increase, and the correcting of aberrations becomes more difficult.
If attempts are made to suitably correct aberrations without increasing the size of the zoom lens system, the number of lens elements are increased. A large number of aspherical surfaces can correct aberrations; however, such an optical arrangement incurs a substantial increase in cost.